This invention relates to disk data storage systems wherein data is stored in sectors of concentric tracks on a rotating disk. More particularly, the invention relates to a system and method for aligning the start of data with the beginning of a sector on such disk storage systems.
In a disk data storage system, regardless of whether such a storage system is a magnetic disk or an optical disk, data is written on or read from the surface as the disk rotates past read/write heads (the term "head" will be used in this disclosure to describe the device which senses the data during a read operation or causes the data to be written during a write operation). Data is typically recorded on the disk in concentric rings called tracks. In a high performance disk data storage system, each track is divided in to a number of segments called sectors. Such sectors provide an identifiable area on the disk where data may be stored and indexed for future retrieval. Data may be stored entirely in a sector, or start at one sector and continue through several sectors. Before a read or write operation can be initiated, the head must be positioned over the desired track, and the disk must be rotated until the desired sector of the selected track is positioned under the head.
The read/write heads are positioned at the correct track by means of a servo system. There are many types of head positioning servo systems known in the art, any one of which could be used with the present invention. There are also may types of Sector positioning systems that could be used to locate the correct sector before reading or writing begins. One such system is described in copending application Ser. No. 472,436 filed Mar. 7, 1983, and assigned to the same assignee as is this application.
When a disk data storage system is writing new data on a disk, the precise location where this data should begin must be accurately determined. When the disk data system is reading data, it also must know the precise location where the disk data is to begin if it is to accurately interpret what is read from the disk. In lower density disk data storage systems, for example floppy disk systems, data starts at a point on the disk called an index. An index defines the start of a circular track as well as the end of the data on that track. In these low density data storage systems data is written consecutively on each track with a gap between each record, and because of the low performance requirements of this type of device, segments are not used. As the performance requirements, such as storage capacity and access time increase, the track must be subdivided into sectors so that data located at various points on the track can be more readily accessed without tying up controller or processing unit resources necessary to search for gaps. The higher density and the division into sectors increases the requirement to determine the exact point where the data starts in each segment.
Where the track is not divided into segments, as in the floppy disk discussed earlier, the beginning of data occurs at the index and thereafter data or gaps are continuous throughout the track. In these low density systems, a long preamble consisting of clocking bits starts near the index and the actual data begins at the end of the clocking bits. Since the index detector and data read/write head are different, the index and start of data may not coincide, however, because of the low performance requirements, mechanical alignment and the use of the preamble, this difference is not significant. In higher density magnetic disk storage systems, data is stored on many recording surfaces arranged as a stack of recording platters much like a stack of phonograph records. In this type of system the tracks are divided into sectors and the sector boundaries are defined on a separate platter known as the servo platter or servo track usually the bottom platter of the stack. There is an individual head for each recording surface, and the heads are all connected together to move in unison. As the heads are positioned over a given location, all tracks are accessible including the servo track, therefore the sector location information recorded on the servo track is used to define sectors on all the other tracks. This system does have an alignment problem between the detection of the start of a sector on the servo platter and the actual start of data on one of the other recording heads. However, another feature of this type of system, the manufacture of the recording heads and the platters as a single unit called a head-disk assembly or HDA, reduces this problem significantly. Since the recording surfaces and the heads that read or write data are always kept as a single unit, a mechanical alignment can be made when the system is manufactured and no alignment is required thereafter. Also, data will always be read by the same head and platter combination that originally wrote the data, further reducing the alignment problem.
As data density increases and with the use of removable media on high density systems, such as an optical disk data storage system, the need for a very precise alignment between the start of a sector and the start of the data in that sector becomes more acute. In an optical disk data storage system that stores over four billion characters on a single surface of a single platter, the density is so high that mechanical alignment alone is insufficient, and electrical alignment must also be performed. With interchangeable media, that is where the data storage disk can be removed from the one disk drive and inserted in another disk drive at a later time, the alignment need is so great that alignment must be performed dynamically each time a disk platter is inserted into a drive. Also, with high density recording on a rotating disk, the alignment changes significantly between the inner diameter of the disk, where the relative motion between the head and the disk is slower, and the outer diameter of the disk, where the relative motion between the disk and the head is much faster.
There exists a need in the art, therefore, for an improved system for aligning the sector boundaries with the start of data within a sector. There is also a need in the art for such a system that can perform the alignment electrically, and also perform the alignment dynamically each time a new disk is inserted into the disk drive. There is a further need for an alignment system that can compensate for the difference between the beginning of a sector and the beginning of data at the inner diameter of a disk, and change this compensation at the outer diameter of a disk. The present invention satisfies these needs.